1. Field of the Invention
This invention relates to semiconductor integrated circuit structures and to methods for manufacturing such integrated circuits wherein self-aligned transistor structures are formed.
2. Description of the Prior Art
The self-aligned silicon gate field effect transistor process and resulting structure has been a standard in the industry for some time. The process involves growing an insulating layer over the semiconductor substrate and then forming a polysilicon layer thereover. Lithography and etching techniques are used to remove all of the polysilicon layer except in the areas where the field effect transistor gates are to be located in the integrated circuit structure. This resulting gate structure is now used as a mask for forming the source/drain of the transistor in what is termed a self-aligned structure.
Improvements have been made in the self-aligned processing and structures. These improvements are particularly important in the manufacture of submicron integrated circuits.
One such improvement field has been for methods to form narrow dimensioned, for example submicron regions on a semiconductor body. Reference is made to the I. T. Ho et al U.S. Pat. Nos. 4,209,349 and 4,234,362, K. Riseman U.S. Pat. No. 4,234,362 and the H. B. Pogge U.S. Pat. No. 4,256,514 which describe the formation of such regions. These patents all involve the formation of substantially horizontal surfaces and substantially vertical surfaces on a silicon body and then forming a layer of a very narrow dimension on both the substantially horizontal and substantially vertical surfaces. This layer is then subjected to an anisotropic etching process such as by reactive ion etching, to substantially remove the horizontal layer while leaving the vertical layer substantially intact. The vertical layer dimension is adjusted depending upon the original thickness of the layer applied. In this way such a narrow dimension region of one micrometer or less is obtained.
Another improved self-aligned gate structure is the lightly doped drain or LDD. For example, where the LDD in an N channel MOS FET is included, in addition to the channel separating implanted N+ source and drain regions, the submicron diffused N- regions. These N- regions increases the channel breakdown voltage and reduces drain junction electron impact ionization (and thus, hot electron emission) by spreading the high electric field at the drain pinch-off region into the N- region. One improved process for making such an LDD device is described in the S. Ogura et al U.S. Pat. No. 4,366,613 in which the N- region is first formed using the polysilicon gate as the mask, submicron sidewall spacers are formed on the sides of the polysilicon gate, and the N+ source/drain regions are formed by ion implantation using the gate and sidewall spacer structure as the mask which results in the N- LDD structure. Other lightly doped drain structures and methods are shown by the I. T. Ho et al U.S. Pat. Nos. 4,209,349 and 4,209,350.
The reverse self-aligned field effect transistor process has been suggested by a limited number of workers in the field. Reference is made to the U.S. Pat. Nos. 4,296,426 to Thomson CSF, 4,378,627 to C. G Jambotkar, 4,419,810 to J. Riseman and 4,546,535 to C. G. Jambotkar. These patents generally describe the reverse process wherein a heavily doped conductive layer such as polycrystalline silicon or the like is formed upon a silicon substrate and an insulator layer formed thereover. This multilayer structure is etched to result in a patterned conductive polycrystalline layer or the like with substantially vertical sidewalls. The pattern of the conductive layer is chosen to be located above the planned source/drain regions with openings in the pattern at the location of the field effect transistor's channel. A sidewall insulator spacer can now be formed on the vertical sidewalls as described, for example in the previous paragraphs. The sidewall layer may be doped with conductive imparting impurities. The gate dielectric is formed on the channel surface. The source/drain region and preferably lightly doped region are simultaneously formed by thermal drive in from the conductive first polycrystalline silicon layer or the like and insulator sidewall layer respectively. The desired gate electrode is formed upon the gate dielectric and electrical connections made to the various elements of the field effect transistor devices.
It is therefor an object of this invention to make improvements in the reverse self-aligned process and resulting devices useful in the submicron integrated circuit technology.
A further object of the invention is to make improvements in the reverse self-aligned process for making submicron LDD MOSFET integrated circuits.
It is still further object of this invention to make improvements in the reverse self-aligned process for making submicron bipolar integrated circuits.